The present invention relates to a process for maintaining open a throttled discharge passage of a sliding closure unit during continuous casting of a molten metal from a metallurgical vessel, such as an intermediate vessel or tundish, through the sliding closure unit to a mold of a continuous casting apparatus. More specifically, the present invention is directed to an improved process for discharging molten metal from a metallurgical vessel through a discharge passage of a sliding closure unit into a continuous casting mold, establishing the molten metal level in the mold within a predetermined range by throttling the passage by a sliding plate of the sliding closure unit, during which throttling operation deposits gradually form in the sliding closure unit to restrict the size of the throttle passage and thereby reduce the molten metal level, and maintaining the molten metal level within the predetermined range by the controlled opening movement of the sliding plate to compensate for restriction of the throttle passage by the formation of such deposits.
U.S. Pat. No. 4,708,193 discloses a process for removing such deposits from the sliding closure unit and to maintain open the throttle discharge passage. This known process involves an initial movement of the sliding plate to close, or at least to restrict, the discharge passage for a substantial period of time to enable the molten metal level in the continuous casting mold to decrease by a substantial amount. The sliding plate then is moved to the completely open position to allow a surge of molten metal to flush away any deposits within the sliding closure unit. This surge of molten metal and deposits fills the space within the mold created by the reduced molten metal level therein.
The present inventor has determined that this decrease of the molten metal level in the continuous casting mold has substantial disadvantageous influences on the quality of the cast strand. It particularly has been determined that such differences in level cause surface defects in the cast strand. The use of this known process, accordingly, discourages the use of such operation at frequent time intervals, such that the process is employed at relatively large time intervals. This in turn tends to reduce the effectiveness of removing deposits from the sliding closure unit, and this itself has a disadvantageous effect on quality of the cast strand.